You have to completely parse the message to extract the signature and then re-serialize the message before you're able to validate the message. Consider a situation where you have a defect in your parser:

- in-line signature: you're applying your parser and serializer to the untrusted body of the message, and then verifying the signature. If this is a malicious payload, you've just run it through your parser and serialzer.

- out-of-message signature: you have the full signature and can verify the message without running a potentially-malicious message through anything other than your signature-verification code.